Fluid quantity indicator

Abstract

An improved fluid quantity indicator, such as for aviation fuel, of the type utilizing capacitive type sensing probes in which a capacitance change occurs proportional to the amount of fluid in a tank due to the dielectric constant of the fluid, such as aviation fuel, being different from that of air, in which the improvement comprises automatic compensation for the zero fluid capacitance variation in a tank and automatic adjustment of the fluid full scale reading for variation in fluid tank capacitance. Programmable digital to analog convertors are utilized to provide a universal apparatus full scale set compensation for containers for the fluid substantially independent of the container configuration for compensating for variation in full scale capacitance of a given container and to provide a universal apparatus zero set compensation for containers for the fluid substantially independent of the container configuration for compensating for variations in zero fluid capacitance of a given container. An up/down counting means together with a digital display is utilized for providing an indication of the fluid quantity, the reference probe signal and the sensing probe signal being preferably summed with the zero set biasing signal in a servo control loop with the counting means for providing the digital display of fluid quantity, the loop being nulled when the counting means output is equivalent to the fluid quantity to provide a container configuration independent real time digital display of fluid quantity within a given container or tank.

What is claimed is: 
     
       1. In an apparatus responsive to the quantity of fluid within a container therefor of a predetermined volume, said apparatus comprising a capacitive sensing means disposed within said container volume so as to be immersible within said fluid and any air within said container volume, said capacitive sensing means having an associated variable dielectric constant, said dielectric constant being proportional to the proportionate amount of fluid within said container volume, said sensing means capacitance varying in accordance with the variation in said porportionate amount of fluid; a reference capacitive means disposed within said container so as to normally be immersible within said fluid and responsive to variations in the dielectric constant of said fluid due to environmental conditions; the improvement comprising AC reference means operatively connected to said sensing means for providing a signal having a predetermined phase and frequency and voltage thereto, a current proportional to the capacitance of said sensing means flowing through said sensing means in response to said provided AC reference means signal; up/down counting means for providing an output signal indicative of said fluid quantity; first computing digital to analog conversion means having a magnitude input operatively connected in parallel to said counting means output for providing an output in response to said input, said conversion means output being operatively connected to said reference capacitive means for providing a voltage thereto, said output comprising a signal having a predetermined voltage and phase and opposite to said AC reference means signal phase and a predetermined frequency identical therewith; second digital to analog conversion means operatively connected to said first conversion means for providing a reference magnitude signal thereto, said reference magnitude signal providing a full scale set for said apparatus, said second conversion means being programmable to provide a universal apparatus full scale set compensation for containers for said fluid substantially independent of said container configuration for compensating for variations in full scale capacitance of a given container; differential amplifier summing means having an input operatively connected to said capacitive sensing means and said capacitive reference means output for summing the output signals provided therefrom; means operatively connected to said summing means output for providing a directional counting signal output in response to said summing means output, said counting signal output being operatively connected to said counting means for counting in response to said counting signal, said counting means output being operatively connected to a digital display means for providing a digital display of said fluid quantity in response to said counting means output; said AC reference means, capacitive sensing means, capacitive reference means, summing means, directional counting signal providing means, counting means and first conversion means comprising a servo control loop which is nulled when said counting means output is equivalent to said fluid quantity, said servo control loop varying said voltage applied to said capacitive reference means for nulling out the current flowing through said capacitive sensing means, whereby a container configuration independent real time digital display of fluid quantity within a given container which is capacitive leakage independent is provided. 
     
     
       2. An apparatus in accordance with claim 1 further comprising third digital to analog conversion means operatively connected to said summing means input for providing a predetermined biasing signal thereto for zero setting said apparatus, said third conversion means being programmable to provide a universal apparatus zero set compensation for containers for said fluid substantially independent of said container configuration for compensating for variations in zero fluid capacitance of a given container. 
     
     
       3. An apparatus in accordance with claim 2 wherein said directional counting signal providng means comprises 90° phase shifter means having its input connected in parallel to said AC reference signal means output for providing said AC reference signal phase shifted 90° as an output therefrom; demodulator means operatively connected to said 90° phase shifter output and said summing means output for providing an output signal therefrom representative of the quadrature current associated with said summing means output; and means operatively connected to said demodulator means output and said AC reference signal output for providing said directional counting signal output in response thereto. 
     
     
       4. An apparatus in accordance with claim 3 wherein said means for providing said directional counting signal output in response to said demodulator means and AC reference signal outputs comprises comparator means for comparing said demodulator means output with a reference for determining direction for said counting signal and gating said direction determined directional counting means, whereby said counting means counts up or down dependent on said determined direction of said gated directional counting signal. 
     
     
       5. An apparatus in accordance with claim 3 further comprising fluid contamination detecting means for detecting resistance current associated with said fluid contamination, said demodulator means eliminating said resistance current from said summing means output, said detecting means being connected in parallel to said summing means output; said detecting means comprising rectifier means for 0° reference demodulating said summing means output to detect said resistance current, threshold detector means for providing an output signal proportional to said resistance current and indicator means for providing an indication of said contamination in response to said threshold detector means output. 
     
     
       6. An apparatus in accordance with claim 2 wherein said second conversion means is operatively connected in parallel to said AC reference signal output and comprises means for providing a 180° phase shift for said AC reference signal as said second conversion means reference magnitude signal, said reference magnitude signal phase and frequency being identical with said conversion means output signal phase and frequency. 
     
     
       7. An apparatus in accordance with claim 2 further comprising fluid contamination detecting means for detecting resistance current associated with said fluid contamination, said detecting means being connected in parallel to said summing means output; said detecting means comprising rectifier means for 0° reference demodulating said summing means output to detect said resistance current, threshold detector means for providing an output signal proportional to said resistance current and indicator means for providing an indication of said contamination in response to said threshold detector means output. 
     
     
       8. An apparatus in accordance with claim 2 wherein said fluid is aviation fuel and said container is a tank therefor. 
     
     
       9. An apparatus in accordance with claim 1 wherein said second conversion means is operatively connected in parallel to said AC reference signal output and comprises means for providing a 180° phase shift for said AC reference signal as said second conversion means for reference magnitude signal, said reference magnitude signal phase and frequency being identical with said first conversion means output signal phase and frequency. 
     
     
       10. An apparatus in accordance with claim 1 wherein said fluid is aviation fuel and said container is a tank therefor. 
     
     
       11. In an apparatus responsive to the quantity of a fluid within a container therefor of predetermined volume, said apparatus comprising a capacitive sensing means disposed within said container volume so as to be immersible within said fluid and any air within said container volume, said capacitive sensing means having an associated variable dielectric constant, said dielectric constant being proportional to the proportionate amount of fluid within said container volume, said sensing means capacitance varying in accordance with the variation in said proportionate amount of fluid; a reference capacitive means disposed within said container so as to normally be immersible within said fluid and responsive to variations in the dielectric constant of said fluid due to environmental conditions; the improvement comprising AC reference means operatively connected to said sensing means for providing a signal having a predetermined phase and frequency and voltage thereto, a current proportional to the capacitance of said sensing means flowing through said sensing means in response to said provided AC reference means signal; up/down counting means for providing an output signal indicative of said fluid quantity; first computing digital to analog conversion means having a magnitude input operatively connected in parallel to said counting means output for providing an output in response to said input, said conversion means output being operatively connected to said reference capacitive means for providing a voltage thereto, said output comprising a signal having a predetermined voltage and phase and opposite to said AC reference means signal phase and a predetermined frequency identical therewith; differential amplifier summing means having an input operatively connected to said capacitive sensing means and said capacitive reference means outputs for summing the output signals provided therefrom; means operatively connected to said summing means output for providing a directional counting signal output in response to said summing means output, said counting signal output being operatively connected to said counting means for counting in response to said counting signal, said counting means output being operatively connected to a digital display means for providing a digital display of said fluid quantity in response to said counting means output; said AC reference means, capacitive sensing means, capacitive reference means, summing means, directional counting signal providing means, counting means and first conversion means comprising a servo control loop which is nulled when said counting means output is equivalent to said fluid quantity, said servo control loop varying said voltage applied to said capacitive reference means for nulling out the current flowing through said capacitive sensing means; and second digital to analog conversion means operatively connected to said summing means input for providing a predetermined biasing signal thereto for zero setting said apparatus, said third conversion means being programmable to provide a universal apparatus zero set compensation for containers for said fluid substantially independent of said container configuration for compensating for variations in zero fluid capacitance of a given container; whereby a container configuration independent real time digital display of fluid quantity within a given container which is capacitive leakage independent is provided. 
     
     
       12. An apparatus in accordance with claim 11 wherein said directional counting signal providing means comprises 90° phase shifter means having its input connected in parallel to said AC reference signal means output for providing said AC reference signal phase shifted 90° as an output therefrom; demodulator means operatively connected to said 90° phase shifter output and said summing means output for providing an output signal therefrom representative of the quadrature current associated with said summing means output; and means operatively connected to said demodulator means output and said AC reference signal output for providing said directional counting signal output in response thereto. 
     
     
       13. An apparatus in accordance with claim 12 wherein said means for providing said directional counting signal output in response to said demodulation means and AC reference signal outputs comprises comparator means for comparing said demodulator means output with a reference for determining direction for said counting signal and gating said direction determined directional counting signal to said counting means, whereby said counting means counts up or down dependent on said determined direction of said gated directional counting signal. 
     
     
       14. An apparatus in accordance with claim 12 further comprising fluid contamination detecting means for detecting resistance current associated with said fluid contamination, said demodulator means eliminating said resistance current from said summing means output, said detecting means being connected in parallel to said summing means output; said detecting means comprising rectifier means for 0° reference demodulating said summing means output to detect said resistance current, threshold detector means for providing an output signal proportional to said resistance current and indicator means for providing an indication of said contamination in response to said threshold detector means output. 
     
     
       15. An apparatus in accordance with claim 11 further comprising fluid contamination detecting means for detecting resistance current associated with said fluid contamination, said demodulator means eliminating said resistance current from said summing means output, said detecting means being connected in parallel to said summing means output; said detecting means comprising rectifier means for 0° reference demodulating said summing means output to detect said resistance current, threshold detector means for providing an output signal proportional to said resistance current and indicator means for providing an indication of said contamination in response to said threshold detector means output.